CMR technology permits communication between the subscribers of a CMR system and subscribers of the public switched telephone network (PSTN). It has been adapted for use in many different applications. For example, CMR equipment may be utilized in automobiles, in portable enclosures, or in remote locations not hard-wired into the PSTN.
In addition, such CMR equipment may be utilized to communicate data as well as voice. Of course, many diverse telecommunications devices, including facsimile machines, have been manufactured to communicate data through the PSTN using local hard-wired loops thereof. However, such devices cannot directly couple to CMR equipment so as to permit communication through such equipment with other subscribers of the PSTN. In large part, this is because a typical telecommunications device does not adequately simulate a CMR control head to a CMR transceiver. Likewise, a typical CMR transceiver does not adequately simulate a central office to the telecommunications device.
A typical telecommunications device is designed to communicate with the PSTN through a two-wire, bidirectional port. Transmitted audio frequency signals, received audio frequency signals, ringing signals (if utilized), and switchhook status detection signals are all communicated through the two-wire port. On the other hand, typical CMR equipment includes a transceiver, a control head, and a handset. Separate transmit, receive, and control signals travel between the control head and handset on one end of a control cable and the transceiver on another end of this control cable. The conventional two-wire, bidirectional communication standard is not used within such CMR equipment. The CMR transceiver interfaces with the PSTN network through electromagnetic radio frequency communications in a manner that is well understood by those skilled in the art and that is largely imcompatible with the above-described two-wire, bidirectional port. Consequently, such a typical telecommunications device is not directly compatible with CMR equipment.
Conventional hybrid circuits which convert two-wire communication into four-wire communication are known to those skilled in the art. Such conventional circuits may utilize inductors, which are highly undesirable due to their cost, tendency to drift, and size requirements. Alternatively, such conventional circuits may require the use of active devices in connection with a resistive bridge structure that operates to cancel out a received signal side tone in a transmitted signal. Such conventional resistive bridge circuits are undesirable because of, inter alia the close tolerances that must be maintained between the associated resistive elements in order to realize the desired cancellation effects. Moreover, a mere hybrid circuit fails to provide a technique for preventing external signals on a transmit leg of a four-wire communication port from contaminating transmission signals derived from a two-wire communication port.
On the other hand, conventional circuits have been constructed which form or thoroughly simulate a telephone network central office to which many telecommunications devices, such as facsimile machines, are designed to couple. While such conventional circuits may work acceptably well in some applications, they tend to be excessively complicated, large, unreliable, and costly for many other applications.
Consequently, a need exists for a simple, inexpensive, reliable circuit that provides sufficient simulation of a central office to a telecommunications device, such as a facsimile machine, to allow such a device to properly operate, while sufficiently simulating a control head to a CMR transceiver.